The present invention relates to a motor vehicle seat, the backrest of which is hinged to the seat cushion, it being possible to adjust and lock the inclination of the backrest in particular angular ranges with respect to the seat cushion by virtue of a special mechanism commonly known as a “hinge”.
The general principle of a seat hinge has been known for a long time, for example from the documents FR 2 599 684 or EP 0 770 514. It is based on the meshing, with a toothed ring, of three sprags at an angular spacing of 120°, it being possible to disengage this meshing by virtue of a hinge pin actuated by a handle. In the disengaged position, the sprags approach the center of the hinge and no longer mesh in the toothed ring.
As an adjustable inclination is only useful in an angular range which is more limited than 120°, and since it is furthermore expedient not to be able to lock the seat outside this angular range, it is known, for example from the document FR 2 659 604, to provide sectors on the toothed ring of the hinge which are not cogged and thus where the sprags cannot mesh. For example, it is known practice to divide each 120° sector of the toothed ring into a cogged 75° sector and a smooth 45° sector. The upper cogged sector is split on either side of the vertical, for example between 65° toward the rear and 10° toward the front; this cogged sector is thus followed toward the front by the smooth 45° sector which is itself followed, at an inclination of 35° to the horizontal, by the start of the next cogged sector.
In some vehicles it is desirable to be able to retract the seat by moving the backrest in folded-flat form toward the front, horizontally. The user in this case has to maneuver the handle of the hinge pin and simultaneously pivot the backrest forward; once the inclination of the latter causes it to enter the sectors of the hinge which are not cogged, that is to say between 10° and 55° beyond the vertical, the user can release the handle since it is no longer possible to lock the seat. However, this locking becomes possible again (beyond 55° in the example given) before reaching the horizontal, such that the user must actuate the maneuvering handle again in the last part of the folding-flat operation in order to prevent the seat from locking before it reaches the retracted position. Moreover, it is difficult for the user to estimate this horizontal position exactly, such that the backrest may in the end be immobilized in a position which is not the absolutely ideal position.